A few years ago I removed both fuel tanks to have them steam cleaned and have access panels installed. I have a honkin big 4 cyl 42hp engine in the way but was able to get them out (and back in). I had remove both cockpit drain hoses and cut the corner off the little (starter battery?) platform molded into the portside aft of the engine liner to get the port tank out but other than that it was just a case of LOTS of twisting & turning, tipping it this way & that to gain ground. Almost like one of those Chinese wood block puzzles that require solving one thing before you can solve another. Not that I ever want to repeat the process but it can be done. 

The only 'advice' I can remember to give is to make use of the fact the step it sits on is more narrow at one end. Keep at it, you'll ultimately figure it out

It was a brute getting the starboard tank in but it was a double brute when the filler nozzle didn't line up under the deck fill port and the tank had to be removed, the nozzle moved to a new position on the tank top, and the tank reinstalled again.  But it's doable, because we did it twice. Ralph

I had a problem with my port side tank filler hose being 'off' about an inch causing a slight bend in the hose (I think it was a later add-in). I had to fill the tank very very slowly or fuel would back splash.

When I removed the tank for cleaning I removed the deck fitting & fuel hose.  Then, once the tank was back in place I sighted thru the hole in the deck to see where the hose flange on the tank was actually positioned relative to the hole in the deck. I had to move the center of the hole in the deck about an inch and I just reversed the resulting moon shaped plug to fill the opposite side of old opening.

You've never confirmed but again I'm assuming the tank you are replacing is the same (or smaller) dimensions as the old?

Unless your engine is bigger than my 42hp 4 cyl Kubota there is no reason to remove the engine.  It may seem impossible but myself and others have done it (and without hacking away at the engine liner). You can you so don't give up.

Unfortunately it was too long ago to remember the steps I took but I do remember being 'confounded' by the width of the 'fat end' of the step the (fat end of the) tank sits on. It required a lot of tilting/rotating/tilting/reverse rotating/sliding back & forth, etc. while (somehow) taking advantage of the steps narrow (aft) end. 

Keep trying... It really is like one of those wire puzzles that cannot be solved except by taking specific steps in a certain order.  You will eventually stumble onto the 'magical set of steps' it takes to get the (actual quote while installing mine) %@#&$ stupid thing in place.  And then you will feel there has never been a more appropriate time to yell 'EUREKA!'

An update on progress if anyone is interested...  

So, we made the cardboard mock-up of the tank and tried it last weekend.  I first tried it full sized – would not fit.  I then cut it down from thirty five inches long to 29 inches long, this allowed it to clear the aft portion of the liner and drop down low enough to tilt back in toward the tank cavity.  The space between the top of lower liner shelf and the bottom of the cockpit only measures 13” and the tank measures 15” – it would not fit into this space.  I then cut the top of the mock-up down to 13” and, with a little persuasion, it fit.  Once I was able to tilt it back into the cavity it slid right up onto the top shelf where it belongs.

In all, I shorten the length of the tank from 35” to 29” and reduced the depth of the tank from 15” to 13”.  To preserve the greatest amount of tank capacity, I took the 6”out of the aft portion of the tank and the 2”off the back/ outboard portion of the tank.  I have no idea how much fuel the tank will hold now but I’d guess somewhere between 25 and 35 gallons.

Because the cardboard tank had more flexibility than the aluminum tank and I only want to do this once, I plan to mock up the new tank dimensions using door skin to increase rigidity and try the process over again.  Assuming the new tank will fit, I’m going to need to either have the existing tank cut down or have a new tank built.

One more try and I think we'll have it nailed down - learned a lot through the process but ready to get 'er down and move on.  I'll update on any others issues and capacity info when it's in the boat and full of fuel.  Don